It is known that the use of PbO as an ingredient in a glass enamel composition tends to lower the firing temperature of the composition and produce an enamel that has a superior surface finish. For this and other reasons, PbO was a significant component in many prior art glass enamel compositions. However, in light of recent health and environmental restrictions and concerns, the use of PbO and other potentially toxic metal oxides, such as CdO, in glass enamel compositions is now largely avoided whenever possible. A need exists in the glass enamel industry for crystallizing and noncrystallizing lead and cadmium free enamel compositions that fuse at low temperatures and form enamels which are resistant to acids, water and alkalis.
One particular application for lead and cadmium free glass enamel compositions that fuse at low temperatures is the formation of opaque, dark colored enamel bands on sections of automotive glass, such as windshields and side and rear windows. Automotive manufacturers have found that the appearance of a section of glass is greatly enhanced by applying a relatively narrow, opaque, dark colored enamel band around one or more edges of a section of glass on the inner surface thereof. This band may vary anywhere from about one inch to about six inches in width. In addition to imparting an aesthetically appealing appearance to the section of glass, these opaque, colored enamel bands preferably block the transmission of sunlight and thereby prevent the degradation of underlying adhesives by ultraviolet radiation (hereinafter "UV"). Moreover, these opaque colored enamel bands preferably conceal a section of the silver-containing buss bars and wiring connections of rear glass defrosting systems from view from the outside of the vehicle.
As noted in Gettys et al. U.S. Pat. No. 4,882,301, the specification of which is hereby incorporated by reference, glass sections for automotive applications are many times produced with varying degrees of curvature as opposed to flat, planar surfaces. If a curvature is desired in a given section of glass, it is heated to a temperature in the vicinity of about 1,300.degree. F. (about 700.degree. C.) at which point it is ready to be subjected to a bending or curving stress employing any number of suitable molding or pressing techniques. At that temperature, the section of glass maintains sufficient stiffness such that the press head of the equipment employed to bend or curve the glass or the vacuum head utilized to pick up and transport the section of glass does not disturb the surface of the glass with which it contacts.
It was discovered several years ago that specially formulated glass enamel compositions could be applied to planar sections of glass to form opaque, dark colored enamel bands at the same time as the bending or forming operations were performed on the section of glass. As set forth in U.S. Pat. No. 4,882,301, these glass enamel compositions had the ability to fuse and partially crystallize at the temperature at which a section of glass would be preheated preparatory to a bending or forming operation. It is believed that the partial crystallization of the enamel forms a dense, hard, protective layer which prevents the enamel from sticking to the press or vacuum head during the glass bending and transporting operations.
Generally speaking, prior art enamel systems suitable for use in such automotive applications fit within one of five broad categories or types. The first category relates to lead and/or cadmium based enamel systems which partially crystallize upon firing. The enamel systems disclosed in U.S. Pat. No. 4,882,301 are representative of this type. The second category relates to lead and cadmium free enamel systems which include crystalline seed materials to promote partial crystallization of the enamel upon firing. The enamel systems disclosed in Ruderer et al. U.S. Pat. No. 5,153,150, Ruderer et al. U.S. Pat. No. 5,208,191, Sakoske U.S. Pat. No. 5,677,251, Sakoske et al. U.S. Pat. No. 5,714,420, Sakoske U.S. Pat. No. 5,753,685, and Sakoske U.S. Pat. No. 5,783,507 are representative of this type.
The third category relates to lead and cadmium free enamel systems which include substantial amounts of Bi.sub.2 O.sub.3, but little if any ZnO. The enamel systems disclosed in Murkens U.S. Pat. No. 5,203,902 and Manabe et al. U.S. Pat. No. 5,578,533 are representative of this type. The fourth category relates to lead and cadmium free enamel systems which include substantial amounts of ZnO, but little Bi.sub.2 O.sub.3. The enamel systems disclosed in Ruderer et al. U.S. Pat. No. 5,306,674, Anquetil et al. U.S. Pat. No. 5,350,718, Emlemdi et al. U.S. Pat. 5,504,045, Heitmann et al. U.S. Pat. No. 5,707,909, and Harada et al. U.S. Pat. No. 5,817,586 are representative of this type. The fifth category relates to lead and cadmium free enamel systems which include both Bi.sub.2 O.sub.3 and ZnO as essential components. The enamel systems disclosed in Roberts U.S. Pat. No. 5,252,521, Ryan U.S. Pat. No. 5,616,417, and Punchak U.S. Pat. No. 5,629,247 are representative of this type.
Although improvements have been made in recent years, the chemical durability of known lead and cadmium free glass enamel systems for automotive applications has been less than desired. Therefore, a need exists for enamel compositions for automotive applications which exhibit excellent chemical durability to acids, water, and alkalis. Such enamel compositions must be able to fuse and preferably, partially crystallize at temperatures at which sections of glass are preheated preparatory to forming operations so as not to stick to press or vacuum heads. Moreover, such enamel compositions should be effective in blocking ultraviolet radiation and in retarding the migration of silver and subsequent showing from overprinted buss bars and wiring connections of rear glass defrosting systems.